Next week brings a milestone in the search for extraterrestrial life with the scheduled launch Friday of NASA's Kepler satellite. The mission, named for 16th- and 17th-century German astronomer Johannes Kepler, will study a group of stars for three-plus years in search of subtle, periodic dips in stellar brightness—the telltale signs of planetary orbits. Although more than 300 planets outside the solar system have already been found using this method, among other techniques, Kepler's strength will lie in its instruments' sensitivity to smaller, cooler planets more hospitable to life and more like our own.

In a new book, planetary scientist Alan Boss, who studies stellar and planetary formation at the Carnegie Institution of Washington and is a member of the Kepler scientific team, argues that the mission should throw open the door to finding life outside the solar system. With any luck, Boss says, Kepler should indicate that billions of habitable planets exist in our galaxy alone, with an almost unfathomable tally of sextillions across the entire universe. (Where are Jules Verne and H. G. Wells when you need them?) We spoke to Boss about the thesis of his book.

[An edited transcript of the interview follows.]

Your new book is called The Crowded Universe. What does the title mean?
The point of the book is to show why one can claim that the universe is likely to be teeming with life. I make the argument throughout the book that we already know that Earths are likely to be incredibly common—every solar-type star probably has a few Earth-like planets, or something very close to it. To my mind, at least, if one has so many habitable worlds sitting around for five billion or 10 billion years, it's almost inevitable that something's going to start growing on the majority of them.

Life is so tenacious and willing to seek out a toehold anywhere it can, my feeling is that it is going to originate anywhere it has a chance. It may not necessarily be little creatures like in a Steven Spielberg movie, but there will be some kind of archaealike or bacterialike microbes crawling around or bubbling along. Those are going to be creating output like oxygen and methane, and those are things we can see in the atmosphere. We may not be able to tell if a planet has intelligent life or dinosaurs, but we can at least tell if it has slime mold. So we're going for the slime mold.

The Kepler satellite, which will seek out those habitable planets, launches March 6. What do you think it is going to tell us?
I'm betting it's going to tell us that Earths are quite common. Kepler's going to be looking at 100,000 stars for three or four years. We expect that Earth-like planets will have their orbits aligned in such a way that basically one out of 1,000 can be seen by Kepler. So if every single star has one Earth-like planet, that means that Kepler will see roughly 100 Earths. But if it turns out that we're wrong and, say, only one out of 100 stars has an Earth-like planet, then Kepler might find one Earth—if it's lucky—or it might find zero. And if Earths only occur around one out of 1,000 stars, we'd have to be really lucky for Kepler to find anything at all.

So Kepler basically assumes that it should be able to find some Earths as long as they occur at roughly a 1 percent rate or higher. There's a very good chance that Earths are available essentially all the time, but we can be a little bit conservative and say that Kepler should find dozens of Earths.

If Kepler does find a number of Earth-like planets, then we'll know how many there are in general because Kepler will have searched such a large sample. And then we'll know how to go about planning the next phase of the search, which is to search the nearby stars. Kepler's going to be searching stars hundreds of light-years away all in one direction of the sky. It's sort of like after looking at another neighborhood in another city, we want to then look at our own block. Kepler will tell us how many houses we have to search on our block to look for life.

Given that our planet is the only Earth we know of, how can we extrapolate how many Earths there should be out there?
First, if you talk to astronomers who look at young stars, they will tell you that when stars form, they tend to have a little bit of angular momentum, which means that they can't accrete all of their matter and they end up having a disk around them. Such disks are what planetary systems form out of, basically the leftovers from the star-formation process. Essentially all young stars have these disks, so we expect that these young stars at least have the possibility of having planetary systems.

Second, those who worry about planet-formation processes find that it's very hard to stop Earth-like planets, or some sort of large, rocky object, from forming. Earths in some sense are easier to build than Jupiters, but we already know from our extrasolar planet census that Jupiters exist around at least 10 percent, and probably around 20 percent, of stars. So Earths should be even more common than that.

Finally, and even more directly, the planetary searches are already beginning to find a new class of planets called super-Earths with masses maybe five, 10 or 15 times the mass of Earth that orbit a little closer to their star than our planet does. These guys occur on roughly one third of nearby solar-type stars. And these are sort of the oddballs in some sense, which I think are very much just the tip of the iceberg of the spectrum of Earth-like planets. In any theoretical model of planet formation that people talk about, there should be a ton of Earths compared to these oddball super-Earths, so when we do a complete census we should find a lot of Earths. If these oddballs are there 30 percent of the time and the Jupiters are there 20 percent of the time, that means the ones we can't quite see should be there essentially all the time. So it's a very compelling story, and all the evidence from several different directions points toward Earths being quite common.

In a recent talk, I see that you gave some rough estimates for how many there might be in the universe.
Yes—very rough estimates. It's pretty straightforward. It's basically saying that every sunlike star should have one Earth, or close to it. And there are roughly 1011, or 100 billion, stars in our galaxy. Most of them are one solar-mass or less, and a lot of them are probably only half a solar mass, but those things probably have Earths orbiting them as well. So I'd say that, within a factor of two or something, roughly every one of those stars should have an Earth-like planet, so you're up to 1011 planets within our galaxy.

And then, well, let's talk about the whole universe. With its deep-field pictures, the Hubble Space Telescope showed—again, very crudely speaking—that there are maybe 1011 galaxies in the observable universe, and presumably those galaxies have planetary systems just like ours does. There's no particular reason to think our galaxy is an oddball, although you might start worrying about elliptical galaxies versus spirals and take out a factor of two or three, but we're playing with rough orders of magnitude here.

So I'd say there are basically 1011 galaxies, and each of them has roughly 1011 stars, meaning 1011 planets, so you're up to 1022 planets (10 sextillion) overall, in very round numbers—and that's a lot of planets. It's mind-boggling but it's inescapable. Even if Earths are a thousand times less frequent than I claim they are, you're down to 1019 (10 quintillion). So is 1019 not big enough for you? I hate saying the word, but these are literally astronomical numbers—numbers that make complete sense in terms of an astronomer's view of the universe but that human beings can't think about except, perhaps, in terms of how much money we're spending on the economic stimulus package. Trillion-dollar bailouts are getting us up toward the right order of magnitude.

Assuming our assumptions of planet formation aren't way off, how long will it take before Kepler turns up some Earths?
We'll find the hot Jupiters quickly and maybe a few hot super-Earths within the first year, I would guess. But the real treasure is the Earth-like planets—and, by definition, you can't get them until you've run for at least three or four years. You're looking for one-year orbital periods, roughly, for solar-type stars, and the first time the transit occurs, you think, "That's interesting." You've got one blip. You need at least two blips to say, "Now I have an orbital period, let's see if there's a third blip at the right time." So if you get the third blip you say, "Hmmm, that looks good, I've got blips that are separated by roughly the same amount of time, maybe I'll just be really cautious and wait for a fourth blip." And if you do that then you're up to a few years of observing time. So my feeling is that by 2013 we'll have some Earths to announce.

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